The present invention relates to a method of treating the surface of magnetic particles in order that the magnetic particles are well dispersed in polymerizable monomers or in polymers when mixed therewith.
In manufacturing magnetic recording materials, magnetic rubber, magnetic plastics and the like, it is required that the magnetic particles to be contained in those materials have surface having affinity for polymerizable monomers, polymers and oily materials in order that the magnetic powder particles can be well dispersed in them. In particular, when toner for use in the developing of latent electrostatic image is prepared, it is one of the essential requirements that the magnetic powder particles have the above-mentioned affinity.
As dry-type developers containing as the main components a coloring agent and a resin for developing latent electrostatic images in electrophotography, electrostatic recording and electrostatic printing, the following developers are conventionally known:
(1) Two-component type developer which is prepared by mixing dry-type toner with carrier particles with a particle size larger than the particle size of the toner particles, thereby triboelectrically charging the toner particles to a polarity opposite to the polarity of latent electrostatic images to be developed. Development by this two-component type developer of the latent electrostatic images is performed by bringing the developer into contact with the latent electrostatic images.
(2) One-component type developer consisting essentially of a toner with relatively low electric resistance or high electric resistance, containing a magnetic material therein. Development of latent electrostatic images by this developer is performed by bringing the developer into contact with or bringing the developer near latent electrostatic images.
Conventionally, when the above-mentioned toners are prepared, a thermoplastic resin is fused and a coloring agent such as a dye or a pigment, a magnetic material, a triboelectric charging control agent, an offset preventing agent, and a lubricating agent are added to the fused thermoplastic resin, and the mixture is well mixed. The mixture is then cooled until it becomes hardened. The hardened mixture is then crushed and pulverized, so that particles with the desired particle sizes are obtained by classification.
However, this method has the following shortcomings:
First, this method requires, for example, a polymerization apparatus for producing the above-mentioned resin, a mixing and kneading apparatus, a crusher or pulverizer and a classifier. Furthermore, due to the use of these apparatus, energy consumption is so great that the cost for the production of the toner is high.
Second, the conditions for uniformly dispersing the above-mentioned components are so delicate that it is extremely difficult to obtain a uniform mixture or dispersion of these components.
Third, in the course of the crushing or pulverizing process in this method, particles with a wide range of particle sizes are obtained. Therefore, a classification process is indispensable. Accordingly, the yield of the particles with the desired particle sizes is relatively low.
Fourth, the obtained particles are so irregular in shape that the fluidity of the particles is poor. Further, since repulverizing of the particles takes place when they are stirred for triboelectric charging, finer particles are produced in the course of the repulverizing and the fogging of the developed images is brought about by those finer particles.
In contrast to this, in Japanese Patent Publication No. 36-10231, Japanese Patent Publication No. 43-10799, Japanese Patent Publication No. 47-51830, Japanese Patent Publication No. 41-14895, U.S. Pat. Nos. 4,264,700 and 4,314,931, there are described methods for producing toner by emulsion polymerization and by suspension polymerization. In these polymerization methods, kneading and crushing processes are unnecessary, and starting materials for producing the toner by these methods are less expensive as compared with polymers employed in conventional methods. Furthermore, since the particles of the toner obtained by these methods are spherical, the fluidity of the toner is much better than that of the toner obtained in the conventional methods. Moreover, the apparatus necessary for these methods is simpler than the apparatuses in the previously mentioned conventional method. Specifically, only one reaction chamber is necessary for the main reaction. The required energy for these methods is much less than the energy as required for fusing, cooling, crushing and classification in the previously mentioned method.
The suspension polymerization method, however, has the shortcoming that, depending upon the degree of the affinity for the magnetic particles of additives added to the resin and of the resin, it may occur that the magnetic particles diffuse from the suspended particles into the aqueous phase or the magnetic particles are not dispersed uniformly within the resin particles. If this takes place, the produced toner cannot fully perform its function.
More specifically, in the suspension polymerization for preparing a one-component type magnetic developer or magnetic toner, a large amount of magnetic powder is dispersed in a polymerizable monomer and the dispersion is suspended in an aqueous dispersion medium with high speed stirring, thereby forming liquid particles containing magnetic particles therein, with almost the same size as the toner particles employed in practice. However, in the course of this dispersing, it takes places that the magnetic particles aggregate at the surface of the liquid particles, the magnetic particles diffuse into the aqueous dispersion medium, or after the polymerization of the liquid particles, the formed toner particles are contaminated by free magnetic particles or the magnetic particles are deposited on the surface of the toner particles, so that the electric conductivity of the toner increases or the surface resistance thereof decreases. As a result, the charging performance of the toner particles is impaired and occasionally image transfer becomes impossible.
In order to improve the dispersion performance of the magnetic particles, it has been proposed to treat the surface of the magnetic particles so as to have an affinity for oils or oily materials. For instance, in Japanese Laid-Open Patent Application No. 57-81271, there is disclosed a method of using magnetic particles together with a titanate coupling agent. In Japanese Laid-Open Patent Application No. 54-84731, there is disclosed a method of treating the surface of magnetic particles with a silane coupling agent in an attempt of preventing the magnetic particles from diffusing into the aqueous phase in the above-mentioned dispersion.
By the above proposed methods, the magnetic particles can be successfully prevented from diffusing into the aqueous phase when dispersing in the aqueous phase a polymerizable monomer containing the magnetic particles therein. However, by these methods it cannot be prevented that the toner particles are exposed at the surface of the toner particles or the magnetic particles are distributed unevenly within the toner particles.
When the above phenomena take place, the surface electric resistance of the toner particles so decreases that it becomes impossible to electrically charge the toner particles and the preservability and resistance to high humidity of the toner are significantly impaired, and the toner particles aggregate, with the result that the fluidity of the toner particles becomes poor.
When the dispersion of the magnetic particles is poor, the liquid particles in the course of suspension polymerization easily collapse. As a result, the apparatus for the suspension polymerization reaction is polluted with the magnetic particles and the advantages of the suspension polymerization are not available.